The ability of a pigmented or filled polymer system to diffuse and reflect a portion of the incident light is known as its scattering power. The scattering power of a pigment or filler is directly linked to its hiding and opacifying power. The scattering power of a filler or pigment is related to two properties, refractive index and particle size distribution. The greater the difference between the refractive indexes of polymer and pigment or filler the greater the scattering power, and therefore the greater the opacity. TiO2 rutile, with the highest refractive index of all common pigments (of the order of 2.55 for anatase and 2.7-2.75 for rutile), is the most efficient scatterer and is well known as an opacifying white pigment for polyolefin polymers, and in particular polyethylene products such as polyethylene film. However, titanium dioxide is an expensive material and so it would be desirable to be able to replace some or all of the titanium dioxide in polyolefin applications. In this respect, one known replacement, or partial replacement, opacifier is particulate calcium carbonate.
U.S. Pat. No. 5,571,851 describes the use of silane treated calcined clays as reinforcing fillers for plastics systems such as polyamides.
Flash calcined kaolin clay, which is made by a process in which a hydrous kaolin clay is exposed to an elevated temperature for a short period of time, for example a few seconds, is already known as a filler for elastomer compositions, see GB-A-2067535. Flash calcined clay has also been used for many years as an extender for titanium dioxide in paints (WO 99/24360).